1. Field of the Invention
The present invention relates to apparatus for digitally multiplexing a plurality of remote, electronically operative devices, and more particularly to a multiplexed apparatus utilizing a surface acoustical wave system.
In an electrical system featuring a number of diverse electronically operated devices, it has been found necessary to locate the switch controls at a remote location. Such a system is frequently to be found in automobiles, wherein the horn, directional signals, lights, wipers, etc., are all controllable from a wand mounted upon the steering wheel column. Other automotive functions such as oil pressure, water temperature and vehicular speed are usually mounted upon a single panel or dash.
In such cases, it may be expensive or inconvenient to provide an individual transmission line to each device or switch. In the aforementioned motor vehicle steering wheel example, manufacturing and reliability problems are posed by passing a great number of wires up a steering column to a rotating member. In such cases, it can be desirable to employ a multiplexing technique whereby the status of more than one switch can be transmitted using a single transmission line. Means such as time division multiplexing, frequency division multiplexing, and packet transmission are well known to those skilled in the art of electrical communications. These well known multiplexing methods require that sophisticated electronic circuits requiring power be installed at both ends of the transmission line.
It would be desirable to replace the electronic circuitry at one end of the transmission line with a passive component not requiring power. Using the steering wheel and tire pressure sensor switch as examples, it would also be desirable to have a simple and reliable means to transmit such data using a noncontacting connector with freedom to rotate.
The present invention has developed a system that can control and/or interrogate remote, electronically operative devices. A single transmission line can carry multiplexed digital information to a pair of coupling inductive coils. One of the coils is associated with a programmable surface acoustical wave device.
A surface acoustical wave transponder that responds to an interrogating pulse with a delayed sequence of pulses, and a circuit for comparing the proper pulse sequence is described in the, aforementioned U.S. patent application Ser. No. 07/483,349. The circuit described therein (see FIG. 8) can be utilized in the present invention to differentiate between switching signals for the remote devices, which switching signals are propagated as reflected surface acoustical waves from the programmable transducers of the SAW device located within the steering wheel column.